a) Field of the Invention
This invention relates to an optical pickup head apparatus for reading information from an information recording medium such as a laser disk.
b) Description of the Related Art
For example, the information saved on a compact disk is stored in the form of bit tracks (tracks) formed on a disk surface. The three beam method and the astigmatism method are commonly used for an optical pickup head apparatus which reads recorded information.
An optical pickup head apparatus using the three beam method or astigmatism method works in the way that a beam emitted from a light source (semiconductor laser) passes through a diffraction grating structured with a holographic optical element and a beam splitter, and, then, is condensed by an object lens; a coherent beam is irradiated at an information track (bit track) of the disk; its reflection is guided to a photodetector comprising a six division photodetector element via the objective lens, beam splitter, and cylindrical lens.
In the three beam method, a beam emitted from a light source is split by the diffraction gratings, into three beams which are the 0 order and .+-.1 order beams and condense them so that three beam spots are arranged at positions slightly angled from an information track. Then, a servo computation is implemented to make the levels of reflection signals (sub-beam signal) from the .+-.1 order spots on sides equal. That is, the sub-beam signals from the .+-.1 order spots are detected by each photodetector element; the difference in output at each photodetector element is computed by subtraction apparatus to obtain a tracking error (TE) signal; the tracking error is servo computed to locate 0 order spot in the center of the track.
The astigmatism method generates astigmatism by the cylindrical lens in the beam of reflected light from the disk surface. A change in spot shape (elliptical direction and elipticallity) caused by misfit focus is computed by adding the outputs of diagonal photodetector elements of a four division photodetector element (the photodetector element which detects the 0 order beam) arranged between a two division photodetector element while obtaining the sum of the output from photodetector elements positioned diagonally by an adding apparatus while obtaining the difference in output from each adding apparatus by a subtracting apparatus to detect a focus error (FE) signal. A radio frequency (RF) signal which is a data signal is obtained by totaling the outputs of the four division photodetector element.
The aforementioned optical pickup head apparatus obtains a radio frequency (RF) data signal by totaling a four division photodetector element such that the center of the signal beam spot, where light beam energy is dense, is located on a split line (dead zone) of the four division photodetector element and the radio signal output fluctuates due to the increase or decrease of the diameter of the spot or due to the spot moving its location. That is, a problem was encountered in that the lens actuator for controlling tracking and focusing invites fluctuation of output signals.